Nondestructive test techniques employing eddy current principles have been used for many years to measure or predict various metallurgical properties of ferrous and nonferrous metals. A periodic magnetic field is applied to a test part to induce eddy currents in the test part, and the amplitude and/or phase of signals induced by the eddy currents in a secondary coil is employed to measure or predict such properties as hardness, case depth, grinder burn, carbon burnout at the material surface, alloy separation and degree of heat treatment. It has also been recognized that, when such techniques are employed in conjunction with ferromagnetic test pieces, the nonlinear magnetization curve of the ferromagnetic material causes odd harmonic frequencies to appear in the output voltage of the secondary test coil. As disclosed in U.S. Pat. No. 4,630,229, amplitude and phase information of the various harmonics, developed by fourier transform or other suitable techniques, can be employed to identify and distinguish various metallurgical properties of the test pieces. Excitation of the test coils at multiple successive frequencies has been found useful to yield additional information on material properties
Although eddy current test systems and methods of the described character have been successfully employed in the laboratory, improvements remain desirable. For example, instrumentation heretofore proposed has generally required operation by a skilled technician for manipulation of test parameters and interpretation of results, and therefore has not been well suited for use in a mass production environment for one hundred percent testing of many pieces per hour. Another disadvantage of prior art devices of the described character lies in general failure to take advantage of current electronic technology, particularly the memory and data manipulation capabilities of available microprocessors, to relieve the operator from the requirement of parameter manipulation and data interpretation during the test process
A general object of the present invention, therefore, is to provide a method and apparatus for measuring properties of test materials, particularly metallurgical properties of ferromagnetic test pieces, that, by employing available digital microprocessor-based technology, are adapted for rapid and accurate measurement of desired properties, and thus are suitable for use by relatively unskilled personnel in the environment of a mass production manufacturing operation. Another and more specific object of the present invention is to provide a method and apparatus of the described character that may be preprogrammed with empirically derived data for controlling test parameters, and thereafter automatically execute a test sequence on successive workpieces without substantial operator intervention. A further object of the invention is to provide a method and apparatus of the described character in which the test parameters and control process may be readily modified or reprogrammed for accommodating changed conditions of the test pieces and/or use in conjunction with test pieces of differing properties. Yet another object of the present invention is to provide a system and method that employs eddy current techniques for measuring properties of test materials at multiple successive test frequencies and/or signal amplitudes in which frequency and/or amplitude change is closely controlled so as to eliminate ringing and generation of spurious harmonics in the secondary test coil, and thereby enhance both the speed and accuracy of the testing operation.
Yet another object of the present invention is to provide a method and apparatus of the described character that is adapted for testing and sorting workpieces that cannot be readily sorted employing conventional technology, such as cast or forged gears, bolts, spindles, camshafts and sprockets. Another and more specific object of the invention is to provide a method and apparatus for enhanced simultaneous sorting for multiple different metallurgical conditions, such as surface hardness, case depth, hardened zone location, material strength, etc. A further object of the invention is to provide a method and apparatus of the described character that employs high power for enhanced sorting of induction-hardened or carburized materials, that are user friendly, that are adapted for communication with industrial controllers or other management information systems for data downloading and statistical evaluation, and that are adapted for automatic one hundred percent in-line material evaluation, as well as evaluation and testing in a laboratory environment.